Polyester resins, especially polyethylene terephthalate (PET) and its copolyesters, are widely used to produce rigid packaging, such as two-liter soft drink containers. Polyester packages produced by stretch-blow molding possess high strength and shatter resistance, and have excellent gas barrier and organoleptic properties as well. Consequently, such plastics have virtually replaced glass in packaging numerous consumer products (e.g., carbonated soft drinks, fruit juices, and peanut butter).
In conventional techniques of making bottle resin, modified polyethylene terephthalate is polymerized in the melt phase to an intrinsic viscosity of about 0.6 deciliters per gram (dl/g), whereupon it is polymerized in the solid phase to achieve an intrinsic viscosity that better promotes bottle formation. Before 1965, the only feasible method of producing polyethylene terephthalate polyester was to use dimethyl terephthalate (DMT). In this technique, dimethyl terephthalate and ethylene glycol are reacted in a catalyzed ester interchange reaction to form bis(2-hydroxyethyl)terephthalate monomers and oligomers, as well as a methanol byproduct that is continuously removed. These bis(2-hydroxyethyl)terephthalate monomers and oligomers are then catalytically polymerized via polycondensation to produce polyethylene terephthalate polymers.
Purer forms of terephthalic acid (TA) are now increasingly available. Consequently, terephthalic acid has become an acceptable, if not preferred, alternative to dimethyl terephthalate as a starting material for the production of polyethylene terephthalate. In this alternative technique, terephthalic acid and ethylene glycol react in a generally uncatalyzed esterification reaction to yield low molecular weight monomers and oligomers, as well as a water byproduct that is continuously removed. As with the dimethyl terephthalate technique, the monomers and oligomers are subsequently catalytically polymerized by polycondensation to form polyethylene terephthalate polyester. The resulting polyethylene terephthalate polymer is substantially identical to the polyethylene terephthalate polymer resulting from dimethyl terephthalate, albeit with some end group differences.
Polyethylene terephthalate polyester may be produced in a batch process, where the product of the ester interchange or esterification reaction is formed in one vessel and then transferred to a second vessel for polymerization. Generally, the second vessel is agitated and the polymerization reaction is continued until the power used by the agitator reaches a level indicating that the polyester melt has achieved the desired intrinsic viscosity and, thus, the desired molecular weight. More commercially practicable, however, is to carry out the esterification or ester interchange reactions, and then the polymerization reaction as a continuous process. The continuous production of polyethylene terephthalate results in greater throughput, and so is more typical in large-scale manufacturing facilities.
When the polymerization process is complete, the resulting polymer melt is typically extruded and pelletized for convenient storage and transportation. Thereafter, the polyethylene terephthalate may be molded into preforms and bottles.
As will be understood by those having ordinary skill in the art, polyethylene terephthalate is typically converted into a container via a two-step process. First, an amorphous bottle preform is produced from bottle resin by melting the resin in an extruder and injection molding the molten polyester into a preform. Such a preform usually has an outside surface area that is at least an order of magnitude smaller than the outside surface of the final container. The preform is reheated to an orientation temperature that is typically 30° C. above the glass transition temperature. The reheated preform is then placed into a bottle mold and, by stretching and inflating with high-pressure air, formed into a bottle. Those of ordinary skill in the art will understand that any defect in the preform is typically transferred to the bottle. Accordingly, the quality of the bottle resin used to form injection-molded preforms is critical to achieving commercially acceptable bottles.
Polyethylene terephthalate bottles, especially straight-walled two-liter soft drink bottles, often possess high coefficients of friction (COF). This is a significant problem in the bottling industry as excessive friction between adjacent bottles prevents such bottles from easily and efficiently sliding past one another as they are depalletized. To improve depalletizing, bottlers conventionally resort to water misting and line lubrication on a filling line.
There is a need for a polyester bottle that possesses reduced coefficient of friction while retaining bottle clarity and, therefore, a corresponding need for a method to achieve the same.